1. Field of the Invention
This invention relates to an integrated circuit structure isolated from portions of a single crystal substrate beneath the integrated circuit structure by an isolation layer of noble gas atoms in the substrate. More particularly, this invention relates to an integrated circuit structure isolated from portions of a single crystal substrate beneath the integrated circuit structure by an isolation layer of noble gas atoms implanted beneath the surface of the substrate and then annealed to form the isolation layer and a method of making such an isolation layer.
2. Description of the Related Art
In the formation of a plurality of integrated circuit structures on/in a common single crystal substrate, e.g., a semiconductor wafer, each integrated circuit structure, e.g., active device, is electrically isolated from other such devices formed on/in the same substrate. Such electrical isolation usually includes lateral isolation which conventionally comprises field oxide or oxide-filled trenches. However, in at least some circumstances, it is also important to provide vertical isolation of the integrated circuit structure from the bulk of the substrate beneath the integrated circuit structure, to avoid coupling through the bulk of the substrate to other devices formed in/on the substrate, to avoid deep punch-through, e.g., by alpha particles, for soft-error reduction, for depletion containment, and to reduce the defect density.
Such vertical isolation can be formed by forming an oxide layer beneath the surface of a substrate by implanting oxygen atoms and then annealing the implanted oxygen to form the desired isolation. Such a structure is usually referred to as silicon-on-insulator isolation. However, while the formation of such an oxide isolation layer beneath the surface of the substrate can result in the reduction or elimination of the aforesaid problems, it is not without problems of its own, including the formation of an electrically floating substrate on/in which the integrated circuit structure will be constructed. This is not desirable because floating substrates are susceptible to bipolar breakdown, reduced noise immunity, and uncertain transistor performance.
Formation of an oxide isolation layer beneath the surface of a substrate is undesirable because the oxygen implant would have to be performed at a high temperature of about 600.degree. C. at very high doses, e.g., 10.sup.18 atoms/cm.sup.2. No resist would protect it, so the whole water would need to be implanted. At the present time there are only a limited number of implanters in existence capable of carrying out such an oxygen implant.
In previous studies of implant dosages of noble gases such as neon, argon, and krypton, into single crystal silicon, the blocking or prevention of epitaxial regrowth of the damaged silicon after implantation by some threshold dosage concentration of the noble gas atoms has been discussed. Cullis, Seidel, and Meek, in an article entitled "Comparative Study of Annealed Neon-, Argon-, and Krypton-Ion Implantation Damage in Silicon", published in the Journal of Applied Physics, 49 (10), October 1978, at pages 5188-5198, discussed implantation damage and noted that for dosages of 6.times.10.sup.15 Ne.sup.+ atoms per cm.sup.2, 2.times.10.sup.15 Ar.sup.+ atoms per cm.sup.2, or 6.times.10.sup.14 Kr.sup.+ atoms per cm.sup.2, each gave a continuous disordered zone from the Si surface to the end of the range after initial implantation, while polycrystalline layers were formed upon annealing at 1100.degree. C. for 30 minutes.
Revesz, Wittmer, Roth, and Mayer, in an article entitled "Epitaxial Regrowth of Ar-Implanted Amorphous Silicon" published in the Journal of Applied Physics, 49 (10), October 1978, at pages 5199-5206, report on the epitaxial regrowth of silicon after implantation of Argon atoms at dosage levels of 2.times.10.sup.15 Ar atoms/cm.sup.2 and 6.times.10.sup.15 Ar atoms/cm.sup.2. They state that initially, the regrowth rate is rather high, but slows down with longer anneals and stops completely after a certain annealing time. They stated that from these facts one might readily conclude that the regrowth is governed by the Ar concentration and that the regrowth stops completely if this concentration reaches a certain amount at the amorphous-crystalline interface.
The same authors (Wittmer, Roth, Revesz, and Mayer) aim published a paper entitled "Epitaxial Regrowth of Ne- and Kr-Implanted Amorphous Silicon" in the Journal of Applied Physics, 49 (10), October 1978, at pages 5207-5212. In this paper they discussed the effect of the presence of such noble gas atoms on bubble growth in the substrate and speculated that epitaxial regrowth can be blocked for the Ne- and Kr-implanted case, as in the Ar-implanted silicon, if the concentration of the implanted specie exceeds a certain critical value.
Aronowitz, in a paper entitled "Quantum-Chemical Modeling of Boron and Noble Gas Dopants in Silicon", published in the Journal of Applied Physics, 54 (7), July 1983, at pages 3930-3934, noted the earlier papers and then calculated that the implanted noble gas atoms would be energetically less stably constrained within the silicon lattice than if free, i.e., not within a lattice structure.
While the presence of noble gas atoms in a silicon substrate is, therefore, not unknown, the previous studies of such centered about the presence of such noble gas atoms in connection with dopant implants, or the presence of such noble gases in gettering, ion etching, and sputtering processes. That is, the presence of such noble gas atoms in the silicon substrate was usually ancillary to some other process or reaction and was, therefore studied either as a tolerated impurity or at least as present or functioning as a supplement to another substance, e.g., a dopant, also present in the substrate.